Bottom Line:
Elevated very low-density lipoprotein (VLDL)-triglyceride (TG) secretion from the liver contributes to an atherogenic dyslipidemia that is associated with obesity, diabetes and the metabolic syndrome.Lastly, we show that the effects of CNS NPY on key liporegulatory proteins are attenuated by hepatic sympathetic denervation.These data support a model in which CNS NPY modulates mediators of hepatic PL remodeling and VLDL maturation to stimulate VLDL-TG secretion that is dependent on the Y1 receptor and sympathetic signaling to the liver.

Affiliation: Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, United States.

ABSTRACT

Objective: Elevated very low-density lipoprotein (VLDL)-triglyceride (TG) secretion from the liver contributes to an atherogenic dyslipidemia that is associated with obesity, diabetes and the metabolic syndrome. Numerous models of obesity and diabetes are characterized by increased central nervous system (CNS) neuropeptide Y (NPY); in fact, a single intracerebroventricular (icv) administration of NPY in lean fasted rats elevates hepatic VLDL-TG secretion and does so, in large part, via signaling through the CNS NPY Y1 receptor. Thus, our overarching hypothesis is that elevated CNS NPY action contributes to dyslipidemia by activating central circuits that modulate liver lipid metabolism.

Results: Manipulation of plasma TG levels in obese ZF rats, through pair-feeding had no effect on liver TG content; however, hepatic PL content was substantially reduced and was tightly correlated with plasma TG levels. Treatment with icv NPY or a selective NPY Y1 receptor agonist in lean fasted rats robustly activated key hepatic regulatory proteins, stearoyl-CoA desaturase-1 (SCD-1), ADP-ribosylation factor-1 (ARF-1), and lipin-1, known to be involved in remodeling liver PL into TG for VLDL maturation and secretion. Lastly, we show that the effects of CNS NPY on key liporegulatory proteins are attenuated by hepatic sympathetic denervation.

Conclusions: These data support a model in which CNS NPY modulates mediators of hepatic PL remodeling and VLDL maturation to stimulate VLDL-TG secretion that is dependent on the Y1 receptor and sympathetic signaling to the liver.

fig4: CNS NPY and Y1 receptor signaling promote expression of ARF-1 and lipin-1. The levels of mRNA and protein of ARF-1 and lipin-1, two key regulatory proteins involved in PL remodeling, were assessed from livers of 4-h fasted lean rats (n = 5–7/group) isolated 60 or 120 min after icv treatment with NPY (black bars), Y1 receptor agonist (gray bars) or Veh (white bars). Relative mRNA levels for liver ARF-1 (A) and lipin-1 (D) were obtained by quantitative RT-PCR. Individual mRNA levels for the protein of interest were first normalized to a non-regulated reference RNA, RPL13A, and then normalized to the Veh group for comparative analysis. Protein extracts prepared from livers of 4-h fasted lean rats isolated 120 min after icv injection of NPY or Y1 receptor agonist or Veh were immunoblotted to detect levels of ARF-1 (B and C) and lipin-1 (E and F). Images of Western blots [representative blots shown for ARF-1 (B) and Lipin-1 (E)] were analyzed by densitometry and data shown relative to Veh levels after normalization to loading controls, β-Tubulin or GAPDH. Data are presented as mean ± SEM and were analyzed by Student's t-test (unpaired, two-tailed); *indicates a significant difference (p < 0.05) between icv treatment and Veh.

Mentions:
We next sought to investigate changes in expression of two key regulatory proteins, ARF-1 [32], [34] and lipin-1 [38], involved in liver PL remodeling as well as VLDL maturation and secretion. We measured mRNA levels at 60 min and protein levels at 120 min post-injection. This later time point (120 min) was included to facilitate the detection by Western blot analysis of any change in ARF-1 or lipin-1 protein levels, since 120 min post-icv injection of NPY and Y1 receptor agonist also elevated plasma TG levels 2-fold above control values [13]. When compared to Veh, icv NPY or Y1 receptor agonist treatment resulted in an approximate 1.7-fold increase in ARF-1 mRNA levels at 60 min post-treatment (Figure 4A). By 120 min, compared to Veh, NPY treatment resulted in a 3.1-fold increase in ARF-1 protein levels (Figure 4B,C). Lipin-1 mRNA levels were induced 3.8-fold, relative to Veh levels, by icv NPY treatment at 60 min post-injection (Figure 4D), and a 2.3-fold increase in protein levels was detected at 120 min (Figure 4E,F). Lipin-1 protein levels were induced 2.2-fold relative to Veh by 120 min post-icv Y1 receptor agonist treatment (Figure 4E,F). Collectively, these data raise the possibility that increased CNS NPY signaling via the Y1 receptor may promote liver PL remodeling and VLDL maturation to enhance VLDL-TG secretion through the modulation of ARF-1 and lipin-1 expression.

Bottom Line:
Elevated very low-density lipoprotein (VLDL)-triglyceride (TG) secretion from the liver contributes to an atherogenic dyslipidemia that is associated with obesity, diabetes and the metabolic syndrome.Lastly, we show that the effects of CNS NPY on key liporegulatory proteins are attenuated by hepatic sympathetic denervation.These data support a model in which CNS NPY modulates mediators of hepatic PL remodeling and VLDL maturation to stimulate VLDL-TG secretion that is dependent on the Y1 receptor and sympathetic signaling to the liver.

Affiliation:
Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, United States.

ABSTRACT

Objective: Elevated very low-density lipoprotein (VLDL)-triglyceride (TG) secretion from the liver contributes to an atherogenic dyslipidemia that is associated with obesity, diabetes and the metabolic syndrome. Numerous models of obesity and diabetes are characterized by increased central nervous system (CNS) neuropeptide Y (NPY); in fact, a single intracerebroventricular (icv) administration of NPY in lean fasted rats elevates hepatic VLDL-TG secretion and does so, in large part, via signaling through the CNS NPY Y1 receptor. Thus, our overarching hypothesis is that elevated CNS NPY action contributes to dyslipidemia by activating central circuits that modulate liver lipid metabolism.

Results: Manipulation of plasma TG levels in obese ZF rats, through pair-feeding had no effect on liver TG content; however, hepatic PL content was substantially reduced and was tightly correlated with plasma TG levels. Treatment with icv NPY or a selective NPY Y1 receptor agonist in lean fasted rats robustly activated key hepatic regulatory proteins, stearoyl-CoA desaturase-1 (SCD-1), ADP-ribosylation factor-1 (ARF-1), and lipin-1, known to be involved in remodeling liver PL into TG for VLDL maturation and secretion. Lastly, we show that the effects of CNS NPY on key liporegulatory proteins are attenuated by hepatic sympathetic denervation.

Conclusions: These data support a model in which CNS NPY modulates mediators of hepatic PL remodeling and VLDL maturation to stimulate VLDL-TG secretion that is dependent on the Y1 receptor and sympathetic signaling to the liver.